1. Field of the Invention
The innovation is a setup in home theater or other sound reproduction equipment to be used for handling low frequency audio signals with an audio signal source producing several digital full bandwidth audio channels and possibly in addition to these one or several digital low frequency sub-woofer channels.
2. Description of the Related Art
As laserdisc players and hifi-VCR's have grown popular, also the so called home theater setups have become widespread, especially in USA. Such a setup typically includes a large screen television, laserdisc player and/or VCR and a sound reproduction system consisting of surround sound decoder, multi-channel audio amplifier and speakers. In addition to the usual two main front speakers there is a center speaker between them, one or two surround speakers situated behind or on sides of the listeners and possibly also a separate subwoofer speaker for reproduction of low bass signals.
A simplified block diagram of a home theater setup is presented in FIG. 1. For the sake of clarity, user and control interfaces, volume control (placed either in digital domain in conjunction with DA-converters or after the bass management) and power amplifiers have been left out of the figure. The setup in FIG. 1 consists of input stage 100, into which both several analog inputs 101, 102, 103 and several digital inputs 104, 105, 106 may be connected. In each case the input signal is selected with signal source selectors 107 and 108, respectively for analog and digital input sources. After this the analog input signal is AD-converted with AD-converter 109 and digital input signal is handled with digital audio receiver 110. From these two digital signals one is selected with source selector 111, followed by the processor unit 112 which will perform the desired processing to the digital audio signal. This processing may include such steps as Dolby Digital (previously Dolby AC-3) decoding, Dolby Pro Logic decoding, THX post processing, room simulations, tone controls etc. After these processing steps these typically six digital audio signals are DA-converted. The number of channels varies between different systems; for instance there may be from 1 up to 4 or 6 surround channels and also there may be more than one center channels. After the DA-conversion there has to be the so called bass management in block 114, to avoid feeding strong bass signals to speakers unable to handle them. As a result, output signals for left front speaker, center speaker, right front speaker, left surround speaker, right surround speaker and subwoofer are generated. These outputs are labelled 115-120 in FIG. 1.
Let us note, that in this context the word speaker refers to a typically enclosed entity consisting of one or several speaker elements. "Large or full range speaker" refers to a speaker capable of reproducing at least most of the audio bandwidth, ie. both low bass signals and high treble sounds. "Small tweeter speaker" refers to a speaker which is only able to reproduce high frequencies and no low bass signals.
There is also low bass material in movie soundtracks. In many cases it is not practical to have six large speakers in a home theater setup. An example of a more practical solution is to have large front main speakers whereas the other speakers are small. Small speakers can not reproduce low frequency signals; high level low frequency signals may even damage these speakers. One of the functions of a home theater device is therefore to extract low bass signals from small speaker output signals and to redirect them to large speakers capable of reproducing them. This function is called bass management (block 114 in FIG. 1).
In most home theater devices bass management is realized with analog filters and switches. This approach is very straight forward to build and is directly compatible with eg. the Dolby configuration definitions. Disadvantages of this approach are the limited flexibility resulting from fixed switches and unclarity from the user's point of view. Therefore most home theater devices can only perform configurations 1 and 2 of the Dolby license definitions, some of them configuration 3 as well. The user can not choose freely from all possible combinations of large and small speakers and also the user has to know and understand the meaning of Dolby configurations 1, 2 and 3 (Dolby AC-3 licensee information manual, version 1.0, figures 9.6, 9.7 and 9.8; all blocks are analog).
The limitations of analog bass management are easy to overcome by carrying out the necessary filtering and summing in digital domain. In this case the user interface for defining the speaker configuration is intuitively: "Front Main: large/small, Center: large/small/none, Surrounds: large/small/none, Subwoofer: yes/no". Now at its best the bass management for one channel may be performed as described in FIG. 2. This entity described in FIG. 2 is unlikely to have been presented before and is placed for each channel in the processing unit 112 in FIG. 1. The result is that the processing unit is only followed by a six-channel DA-converter 113 while bass management unit 114 is left out as obsolete.
In FIG. 2 the incoming digital signals for left front speaker, center speaker, right front speaker, left surround speaker, right surround speaker and subwoofer speaker have been marked 201-206. These signals are fed to complementary filters 207-211, which divide the incoming signals to "high" and "low " output signals, some of which equals the incoming signal except for slight processing delay. These ten output signals as well as the subwoofer output signal are driven through adjustable amplifying stages 212-222 to summing stage 223 to generate output signal 224 for one speaker. With summing stages 212-222 the output signal for each speaker may be flexibly set as combination of full bandwidth input signals.
Controlled by speaker configuration switches a large speaker gets both "high " and "low" outputs. A small speaker only gets "high" output signals, which lack the low bass sounds. These will be directed to large speakers. Also leaving a speaker completely off the design is possible. In this case the signal of eg. missing center speaker may be divided between the two main front speakers. Also when bass sounds are divided to several speakers for reproduction (eg. configuration with several large speakers but no subwoofer) the correct level for bass sounds can be set.
The benefits of this approach also include perfect control of processing delays and thus the signal delays experienced by the listener, and the fact that the sound field may be controlled by peculiar ways eg. by shifting it 90 or 180 degrees.
In practice some summing coefficients are always zero eg. right channel signals are never redirected to left channel, and subwoofer never gets any high sounds. To save processing power the system described in FIG. 2 needs not to be implemented completely. It may also be carried out with appropriate combinations of ordinary non-complementary filters in which the signal is suitably fed, even if the processing block diagram would be substantially different from the one described above.
Even though digital bass management has many benefits it also has its drawbacks especially when using Dolby AC-3 system (also known as Dolby Digital). This system includes a separate subwoofer channel which by definition is to be reproduced ten desibels (10 dB=3.16) louder than the other channels. Since the dynamic range of DA-converters is limited the sound quality may then deteriorate. Let us assume for example a speaker configuration with large main front speakers, small other speakers and no subwoofer speaker in a situation in which the incoming signal has full amplitude bass sound in every channel. If reference level for full amplitude signal is said to be 1 then the summed bass signal level of surround speakers is 2, center channel bass signal level is 1 and subwoofer channel signal level is 3.16, and the summed redirected bass signal level is thus 3.16+2+1=6.16. When this is divided equally to left and right main front speakers the signal level of these channels becomes 1+(6.16/2)=4.08=4.08=+12.2 dB. In order to avoid overdriving the DA-converters under any circumstances the signal level has to be reduced 12.2 desibels before DA-converters. The worst case is a speaker configuration with no subwoofer and in which only the center channel is large resulting in a need to reduce the signal level by 18.2 dB. This leads to deterioration of sound quality.
The 18 or 20 bit DA-converters currently widely in use have dynamic range of about 85-95 dB. When signal level is dropped before DA-converters their limited dynamic range is not fully exploited (12-18 dB equals 2-3 bits).
It is also inconvenient for the user if the output signal level varies substantially between different speaker configurations. For instance determining correct amplifying power and sensitivity is almost impossible.